356 ALASKAN GLACIER STUDIES constricted, and might have moved fastest and eroded most efficiently there is a reef in midchannel (Middle Rock) which was not removed by glacial erosion, and which rises just above sea level at all stages of tide. The contours (Figs. 26 and 28) show that the part below sea level has the typical roches moutonn£es form with the gentler slope on the side from which the ice came. South of it the fiord bottom is basined, probably by glacial erosion, and similar basining is also developed north of Valdez Narrows where the whole of the inner Port Valdez is 700 to 800 feet deep in contrast with 400 to 600 feet at the Narrows. Outside the Narrows, also, Valdez Arm seems to be basined, for the fiord is 1200 to 1250 feet deep but separated from outer Prince William Sound by a broad 200-foot barrier between Pt. Freemantle and Bligh Island. This may be either a submerged moraine or an uneroded reef. Galena and Jack Bays also have a basined character which may be explained either by bottom deposits or by differential glacial erosion. So far as known there are no submerged moraines, except the one at the entrance to Shoup Bay. How much the fiord was filled by glacial deposits is not known, but the delta at the head of the fiord, built by the streams on the outwash plain in front of Valdez Glacier and by Lowe River, has doubtless encroached considerably upon the ice-eroded fiord and is still advancing at a rapid rate. Possibly also the flat floor of the fiord has been in part produced by glacial deposit. The fine mud and silt, shown on the Coast Survey chart, slopes only about 50 feet to the mile after leaving the steep descent of the submerged delta front near the town of Valdez.